Optical filters

ABSTRACT

A hybrid film, comprising a first polymer film having a plasma-treated surface and a second polymer film having first and second surfaces, with the first surface of the second polymer film being disposed along the first plasma-treated surface of the first polymer film has superior thermal and mechanical properties that improve performance in a number of applications, including food packaging, thin film metallized and foil capacitors, metal evaporated magnetic tapes, flexible electrical cables, and decorative and optically variable films. One or more metal layers may be deposited on either the plasma-treated surface of the substrate and/or the radiation-cured acrylate polymer. A ceramic layer may be deposited on the radiation-cured acrylate polymer to provide an oxygen and moisture barrier film. The hybrid film is produced using a high speed, vacuum polymer deposition process that is capable of forming thin, uniform, high temperature, cross-linked acrylate polymers on specific thermoplastic or thermoset films. Radiation curing is employed to cross-link the acrylate monomer. The hybrid film can be produced in-line with the metallization or ceramic coating process, in the same vacuum chamber and with minimal additional cost.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is a continuation-in-part application ofSer. No. 08/334,739, filed Nov. 4, 1994.

TECHNICAL FIELD

[0002] The present invention relates generally to thin metallized andnon-metallized polymer films that incorporate additional coatings andsurface functionalization, which impart application specific propertiessuch as improved thermal stability, abrasion resistance, moisturebarrier and optical effects, that make these films useful in foodpackaging applications, electrical applications that include filmcapacitors and cables, metal evaporated magnetic tapes, printing,decorative wraps and optically variable films for security applications.

BACKGROUND ART

[0003] Metallized and non-metallized films are commonly used in avariety of electrical, packaging and decorative applications. Althoughthe application field is quite broad, the desired properties of thedifferent films are basically the same. These desired properties includemechanical strength, thermal and chemical resistance, abrasionresistance, moisture and oxygen barrier, and surface functionality thataids wetting, adhesion, slippage, etc. As a result, a multitude ofhybrid films have been developed to service a wide range ofapplications.

[0004] In general, hybrid metallized and polymer coated films utilize avariety of production methods. For example metallized polymer films areusually corona-, flame-, or plasma-treated, to promote adhesion of themetal to the polymer surface (U.S. Pat. Nos. 5,019,210 and 4,740,385);or ion beam-treated and subsequently electron beam-charged to promoteadhesion and flattening of the film onto a substrate by electrostaticforce (U.S. Pat. No. 5,087,476). Polymer coatings that serve variousfunctions such as printability, adhesion promotion, abrasion resistance,optical and electrical properties, have been produced using varioustechniques that include thermal cure, reactive polymerization, plasmapolymerization (U.S. Pat. No. 5,322,737), and radiation-curing usingultra-violet and electron beam radiation (U.S. Pat. Nos. 5,374,483;5,445,871; 4,557,980; 5,225,272; 5,068,145; 4,670,340; 4,720,421;4,781,965; 4,812,351; 4,67,083; and 5,085,911). In such applications, amonomer material is applied using conventional techniques of rollcoating, casting, spraying, etc., and the coating is subsequentlypolymerized under atmospheric pressure conditions is More recently, anew technique has been developed that allows the formation ofradiation-curable coatings in the vacuum using a flash evaporationtechnique that leads to the formation of a vapor-deposited thin liquidmonomer which can be radiation-cured (U.S. Pat. Nos. 4,842,893;4,954,371; and 5,032,461 and European Patent Application 0 339 844).This technique overcomes the limitations of conventional techniques forapplying the liquid monomers and requires relatively low doses ofradiation for polymerization.

[0005] The vacuum polymer coating technique as described in the abovereferences was found to have some critical limitations on certainmechanical, thermal, chemical and morphological properties that canreduce their usefulness in packaging films, capacitors, metal evaporatedmagnetic tapes and optically variable films. The invention disclosedherein overcomes these problems and extends the one-step polymer andmetal vacuum coating technique to new functional products with a uniqueset of properties.

DISCLOSURE OF INVENTION

[0006] It is an object of the present invention to produce a hybridpolymer film that has superior mechanical, thermal, chemical and surfacemorphological properties. In conjunction with one or more metal coatingsor a ceramic coating, the hybrid film can be used to produce improvedpackaging films, film capacitors, metal evaporated magnetic tapes andoptically variable films.

[0007] It is also an object of this invention to produce hybrid filmswith controlled surface microroughness. This includes films that have aflatter surface than that of the base film, or a surface with controlledmicroroughness.

[0008] It is another object of the invention to provide an improvedprocess for applying, polymerizing, and discharging, one or more layersof vacuum-deposited radiation-curable monomer films that are used toproduce the hybrid polymer film in a one-step continuous process.

[0009] In accordance with the present invention, a hybrid polymer filmcomprises a first polymer film having a plasma-treated surface and asecond polymer film having first and second surfaces, the first surfaceof the second polymer film being disposed along the first plasma-treatedsurface of the first polymer film.

[0010] The base, or first polymer, films used in the invention toproduce the hybrid films are chosen from a group of thermoplastic filmsthat include polypropylene, polyethylene terephthalate, high and lowdensity polyethylene, polycarbonate, polyethylene-2,6-naphthalate,nylon, polyvinylidene difluoride, polyphenylene oxide, and polyphenylenesulfide, and thermoset films that include cellulose derivatives,polylmide, polyimide benzoxazole, and polybenzoaxozole. The secondpolymer films are radiation-polymerized monomer films that aremultifunctional acrylate or acrylated monomers that contain double bondscapable of radical polymerization. Plasma treatment with gases from thegroup of N₂, Ar, Ne, O₂, CO₂, and CF₄ is used to functionalize the basefilm, to further improve the cross-linking of the acrylate film surface,and to remove surface charge, which improves winding and unwinding ofthe hybrid film. Inorganic layers may be used in combination with thepolymer layers to produce different end use hybrid films; such inorganiclayers include metals selected from the group consisting of aluminum,zinc, nickel, cobalt, iron, iron on aluminum, zinc on silver, zinc oncopper, and zinc on aluminum, nickel-cobalt alloys, andnickel-cobalt-iron alloys, and ceramics selected from the groupconsisting of aluminum oxide, silicon oxides (SiO_(x), where x=1 to 2),tantalum oxide, aluminum nitride, titanium nitride, silicon nitride,silicon oxy-nitride, zinc oxide, indium oxide, and indium tin oxide.

[0011] The hybrid polymer film evidences both improved corrosionresistance and current carrying ability of metallized capacitorscompared to prior art polymer films and overall reliability in demandingapplications that require operations in extreme conditions of voltagecurrent and temperature.

[0012] As incorporated in food packaging, the presence of the acrylatepolymer on a thermoplastic polymer such as polypropylene improves theoxygen and moisture barrier of metallized and ceramic coated films, andit also improves the mechanical properties of the barrier layer to theextent that there is less damage of the barrier layer as a function offilm elongation.

[0013] By adjusting the chemistry of the acrylate coatings, the surfaceof the hybrid films can be made hydrophobic/philic, oliophobic/philicand combinations thereof. This can accommodate different printing inksfor packaging film applications in addition to the improvement ofbarrier properties. Such metallized printable film produced in aone-step process eliminates the lamination of an additional polymer filmthat is used to protect the metal layer and provide a printable surface.

[0014] The hybrid films can have reduced surface microroughness, thuseliminating the need for costly flat films for magnetic tapeapplications. Increased and controlled surface microroughness on ahybrid film can result in lesser abrasion damage and the formation ofunique interference effects cause color shifts with changing viewingangle.

[0015] As incorporated in electrical flexible cables, fluorinatedacrylate polymers deposited on such thermoset polymer films aspolyimide, polyimide benzoxazole (PIBO), and polybenzoaxozole (PBO),prevent electrical tracking, and only carbonize in the presence ofelectrical arcing.

[0016] Color shifting effects useful in decorative and securityapplications can be produced in a one-step low cost process by properchoice of the thickness of the metal and polymer layers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic of apparatus useful for carrying out theprocess of the invention;

[0018]FIG. 2, on coordinates of electrical resistance (in ohms) and time(in hours), is a plot showing changes in electrical resistance due tothe corrosion of aluminum deposited on control polypropylene (PP) filmand on PP/plasma/acrylate hybrid film, in which the aluminum metallizedfilms were exposed to temperature and humidity ambient conditions of 70°C. and 85% relative humidity, respectively;

[0019] FIGS. 3A-E depict various combinations of films that may bedeposited on a film substrate, including an acrylate film on thesubstrate (FIG. 3A), a metal film on an acrylate film on the substrate(FIG. 3B), an acrylate film on a metal film on an acrylate film on thesubstrate (FIG. 3C), an acrylate film on a metal film on the substrate(FIG. 3D), and both sides of the substrate coated with an acrylate film(FIG. 3E);

[0020]FIG. 4, on coordinates of increase in electrical resistance (inpercent) and elongation (in percent), plots the electrical resistanceincrease as a function of elongation, which is an indication ofresistance to corrosion, for an untreated metallized polymer film andfor a metallized polymer film that is plasma-treated;

[0021]FIG. 5, on coordinates of increase in electrical resistance (inpercent) and elongation (in percent), plots the electrical resistanceincrease as a function of elongation for an untreated metallized polymerfilm and for a metallized polymer film that is plasma-treated andcoated;

[0022]FIG. 6, on coordinates of number of samples and oil/water index,plots the extent of wetting by oil- and aqueous-based liquids onpolymeric samples;

[0023]FIG. 7A is a cross-sectional view of an acrylate-coatedpolypropylene film in which the surface of the acrylate coating ismicro-roughened;

[0024]FIG. 7B is a cross-sectional view of the coated polymer film ofFIG. 7A, following deposition of a thin metal layer on themicro-roughened surface of the acrylate coating; and

[0025]FIG. 8 is a cross-sectional view of the hybrid polymer film of theinvention configured for optical filter applications.

BEST MODES FOR CARRYING OUT THE INVENTION

[0026] The vacuum polymer coating technique as described in U.S. Pat.Nos. 4,842,893; 4,954,371; and 5,032,461 and European Patent Application0 339 844 was found to have some critical limitations that include thefollowing: (a) when an acrylate monomer material is deposited on apolymer film substrate, adsorbed oxygen on the surface of the filmscavenges radiation induced free radicals and inhibits thepolymerization process in the interface area; (b) contaminants and lowmolecular weight species on the surface of most polymer materials caninhibit the wetting of the vapor deposited liquid monomer, resulting inthin polymer films with poor uniformity; (c) when this process is usedto produce coatings on a polymer web that is moving at high speed, thepolymer coating does not always reach 100% polymerization; and (d) whenelectron radiation is used to cure the acrylate monomers, electronstrapped on the surface of the film cause electrostatic charging. Thecombination of partial cure and electrostatic charge (trapped electrons)on the film surface, can cause the film to block or stick to itself whenit is wound into a roll.

[0027] The discussion below is directed in the main to a hybrid filmcomprising polypropylene (PP) coated with a vacuum-deposited,radiation-curable acrylate monomer film that is polymerized upon curing.However, it will be understood that this discussion is exemplary only,and is not intended to be limiting to the composition of the coatedpolymer or to the presence or absence of a metal coating on the hybridfilm.

[0028] The hybrid film comprises PP film coated with a high temperature,cross-linked, acrylate polymer, deposited by a high speed vacuumprocess. The basic aspects of the process are disclosed in U.S. Pat.Nos. 4,842,893; 4,954,371; and 5,032,461. However, that process ismodified for the purposes of the present invention.

[0029]FIG. 1 depicts an example of apparatus 10 suitably employed in thepractice of the present invention. A vacuum chamber 12 is connected to avacuum pump 14, which evacuates the chamber to the appropriate pressure.The essential components of the apparatus 10 within the vacuum chamber12 include a rotating drum 16, a source spool 18 of polymer film 20, atake-up spool 22 for winding the coated polymer film 20′, suitable guiderollers 24, 26, a monomer evaporator 28 for depositing a thin film of anacrylate monomer or mixture containing an acrylate monomer on thepolymer film, and radiation curing means 30, such as an electron beamgun, for cross-linking the acrylate monomer to form a radiation-curedacrylate polymer.

[0030] Optionally, an evaporation system 32 for depositing an inorganicfilm on the acrylate film may be employed. Also optionally, a secondmonomer evaporator 128 and radiation curing means 130 may be situatedafter the resistive evaporation system 32. These optional aspects arediscussed in greater detail below.

[0031] The vacuum employed in the practice of the invention is less thanabout 0.001 atmosphere, or less than about 1 millibar. Typically, thevacuum is on the order of 2×10⁻⁴ to 2×10⁻⁵ Torr.

[0032] In operation, the polymer film 20 is fed from the source spool 18onto the rotating drum 16, which rotates in the direction shown by arrow“A”, via guide roller 24. The polymer film passes through severalstations, is picked off from the surface of the rotating drum 16 byguide roller 26, and is taken up by take-up spool 22 as coated film 20′.As the polymer film 20 is taken off the source spool 13, it passesthrough a first plasma treatment unit 36, where the surface of the filmto be coated is exposed to a plasma to remove adsorbed oxygen, moistureand any low molecular weight species from the surface of the film priorto forming the acrylate coating thereon. Just before the coated polymerfilm 20′ is wound on the take-up spool 22, it passes through a secondplasma treatment unit 38, where the coated surface of the film isexposed to a plasma to finish curing the acrylate coating and to removeany accumulated electronic charge.

[0033] The conditions of the plasma treatment are not critical, and theplasma source may be low frequency RF, high frequency RF, DC, or AC.

[0034] The rotating drum 16 is a water-cooled drum driven by a motor(not shown). The drum 16 is cooled to a temperature specific to theparticular monomer being used and generally in the range of −20° to 50°C. to facilitate condensation of the monomer (in vapor form). The drum16 is rotated at a surface speed within the range of 1 to 1000cm/second.

[0035] The polymer film 20 may comprise any of the polymers that havethe requisite properties to be treated as described below. Examples ofsuch polymers include the thermoplastic polymers such as polypropylene(PP), polyethylene terephthalate (PET), polycarbonate,polyethylene-2,6-naphthalate, polyvinylidene difluoride, polyphenyleneoxide, and polyphenylene sulfide, and the thermoset polymers such aspolyimide, polyimide benzoxazole (PIBO), polybenzoaxozole (PBO), andcellulose derivatives.

[0036] The acrylate monomer is deposited on the polymer film 20 by themonomer evaporator 28, which is supplied with liquid monomer from areservoir 40 through an ultrasonic atomizer 42, where, with the aid ofheaters (not shown), the monomer liquid is instantly vaporized, i.e.,flash vaporized, so as to minimize the opportunity for polymerizationprior to being deposited on the polymer film 20. The specific aspects ofthis part of the process are described in greater detail in theabove-mentioned U.S. Pat. Nos. 4,842,893; 4,954,371; and 5,032,461 anddo not form a part of the present invention.

[0037] The flash-vaporized monomer condenses on the surface of thepolymer film 20 that is supported on the cooled rotating drum 16, whereit forms a thin monomer film.

[0038] The condensed liquid monomer is next radiation-cured by theradiation curing means 30. The radiation curing means may comprise anyof the common methods for opening the double bonds of the acrylatemonomer; examples of suitable means include apparatus which emitelectron beam or ultra-violet radiation. Preferably, theradiation-curing means 30 comprises a thermionic or gas dischargeelectron beam gun. The electron beam gun directs a flow of electronsonto the monomer, thereby curing the material to a polymerized,cross-linked film. Curing is controlled by matching the electron beamvoltage to the acrylate monomer thickness on the polymer film 20. Forexample, an electron voltage in the range of about 8 to 12 KeV will cureabout 1 μm thick of deposited monomer. As with the specifics regardingthe deposition of the acrylate monomer, the specific aspects of thispart of the process are described in the above-mentioned U.S. Pat. Nos.4,842,893; 4,954,371; and 5,032,461, and do not form a part of thepresent invention.

[0039]FIG. 3A depicts the coated film 20′ at this stage, comprising anacrylate polymer film 122 on a polymer film web or substrate 120. Such acoated film 20′ has a variety of uses, including high temperatureelectrical cables and foil capacitors in which the film is wound with ametal foil.

[0040] The cured acrylate monomer, or cross-linked polymer, then passesto the optional resistive evaporation system 32, where an inorganicmaterial, such as aluminum or zinc, can, if desired, be deposited on thecured monomer layer. Two such coated films 20′ may be wound to formmetallized capacitors. FIG. 3B depicts a coated film 20′ with ametallized layer 124 on the acrylate polymer film 122.

[0041] The same material may be used for food packaging films,preferably with an additional acrylate coating over the aluminum metallayer to protect the thin metal layer and thus improve the barrierproperties of the film. FIGS. 3C and 3D depict two alternateconfigurations useful in food packaging. In FIG. 3C, the additionalacrylate coating 122′ is formed on top of the metal film 124, while inFIG. 3D, the metal film is first deposited on the polymer film substrate120, followed by deposition of the acrylate film 122 thereon. Theprocess for forming either of these configurations is discussed ingreater detail below.

[0042] The resistive evaporation system 32 is commonly employed in theart for metallizing films. Alternatively, other metal depositionsystems, such as a conventional electron beam vaporization device, andmagnetron sputtering may be employed. The resistive evaporation system32 is continually provided with a source of metal from the wire feed 44.

[0043] The deposition of the metal film may be avoided, therebyproviding simply a hybrid polymer film, which may have a variety ofuses, as described above, with reference to FIG. 3A.

[0044] Following the optional metallization stage, a second, optionalacrylate monomer deposition may be performed, using monomer evaporator128 and radiation-curing means 130. This second deposition is used toform the coated films 20′ shown in FIGS. 3C and 3D, discussed above. Thecomposition of the second acrylate film 122′ may the same or differentas that of the first acrylate film 122.

[0045] The apparatus shown in FIG. 1 is primarily directed toward theformation of an acrylate film 122, 122′ on top of another polymer film120, with or without a layer of metal 124. Further, the uncoated side ofthe polymer film 20 could also be coated with an acrylate film 222 ofthe same or different composition, for example, by providing a secondrotating drum within the vacuum chamber 12 and providing the samesequence of devices (monomer evaporator 28 and radiation curing means30, with optional metallization device 32). Such a coated film 20′ isdepicted in FIG. 3E.

[0046] The acrylate polymer film 122, 222 is formed to a thicknesswithin the range of about 0.01 to 12 μm, depending on the particularapplication. While thicker films than this may be fabricated, no benefitseems to be derived using such thicker films.

[0047] The thickness of the polymer film substrate 20, 120 is typicallywithin the range of about 1 to 100 μm, again, depending on theparticular application.

[0048] When used with conventional polymer films employed in metallizedcapacitors, such as PP, PET, or polycarbonate, the thickness of theacrylate film 122, 222 is typically on the order of 0.1 to μm. In suchcases, the underlying base film (PP or polycarbonate) 20, 120 is muchthicker; commercially available films of PP are in the range of 4 to 25μm. Metallized thin film PP capacitors are used in low loss, ACapplications, and the presence of the acrylate film provides a number ofadvantages, including greater reliability and, unexpectedly, improvedcorrosion resistance. The dielectric constant of the acrylate film foruse in such applications preferably is in the range of about 2.5 to 4.0.

[0049] On the other hand, there are applications requiring energystorage in which higher dielectric constants in the range of about 10 to15 are desired. In such cases, acrylate polymers having such a highdielectric constant are deposited on thinner films, and thus theacrylate film comprises a substantial fraction of the hybrid filmthickness.

[0050] In packaging applications, the acrylate coatings 122, 222 aretypically about 0.5 to 2.0 μm thick, deposited on PP or PET films 20,120 that are typically 12 to 35 μm thick.

[0051] Examples of acrylate monomers that may be used in the practice ofthe present invention include ethoxylated bis-phenol diacrylate,hexadiol diacrylate, phenoxyethyl acrylate, acrylonitrile,2,2,2-trifluoromethyl acrylate, triethylene glycol diacrylate, isodecylacrylate, alkoxylated diacrylate, tripropylene glycol diacrylate, ethoxyethyl acrylate, polyethylene glycol diacrylate, diethylene glycoldiacrylate, trimethylol propane triacrylate, tetraethylene glycoldiacrylate, cyano-ethyl (mono)-acrylate, octodecyl acrylate, dinitrileacrylate, pentafluorophenyl acrylate, nitrophenyl acrylate, iso-bornylacrylate, tris(2-hydroxyethyl)-iso-cyanurate triacrylate,tetrahydrofurfuryl acrylate, neo-pentyl glycol diacrylate, propoxylatedneo-pentyl glycol diacrylate, and mixtures thereof.

[0052] The metal film 124 that is deposited in the case of metallizedcapacitors may comprise aluminum or zinc or a composite film of nickelon aluminum, iron on aluminum, zinc on silver, zinc on copper, or zincon aluminum. The thickness of the metal film 124 for capacitor use is onthe order of 100 to 300 Å, and may be uniform across the width of thehybrid film 20′ or may be thicker at the edges than in the centralportion.

[0053] The metal film 124 that is used in packaging typically comprisesaluminum. The thickness of the metal film is within the range of about100 to 400 Å.

EXAMPLES

[0054] I CAPACITORS

[0055] Capacitors employ low temperature thermoplastic dielectric thinfilm polymers, such as polypropylene (PP), polyethylene terephthalate(PET), polycarbonate, polyethylene-2,6-naphthalate, polyvinylidenedifluoride (PVDF), polyphenylene oxide, and polyphenylene sulfide,either metallized or maintained between metal foil electrodes.Metallized film capacitors are used extensively in a broad range ofelectrical and electronic equipment that include motor run and motorstart circuits for air conditioners, fluorescent and high intensitylight ballasts, power supplies, telecommunication equipment,instrumentation, and medical electronics. In many of these applications,the metallized capacitors are used to conserve energy by correcting thepower factor of a circuit and in others they are used to performspecific functions, such as timing, filtering, and decoupling. Theadvantages of metallized film over film foil capacitors include lowervolume, weight, cost, and higher reliability due to the self-healingproperties of the metallized films. The low temperature metallized filmdielectrics find use in such a multitude of high voltage and highfrequency capacitor applications due to their low dielectric loss.

[0056] One disadvantage, however, is lower current-carrying capacity dueto the thin metallized electrodes that are deposited on low temperaturethermoplastic dielectrics, such as PP, PET, and polycarbonate. Inparticular, reliability issues arise when metallized capacitors areforced to carry high levels of pulse or AC current. The presentinvention addresses the production of a hybrid film that has improvedmechanical and thermal properties that improve capacitor performance andreliability.

[0057] The following discussion is directed to low loss, ACapplications, involving the deposition of an acrylate film on apolypropylene film, followed by metallization to form a thin filmmetallized capacitor. It will be readily apparent to the person skilledin this art, however, that these teachings may be extended to other basepolymer films for other applications.

[0058] Over 100 acrylate monomer materials have been polymerized andtested for dielectric constant and dissipation factor as a function oftemperature and frequency, and oil and water surface wetting. Some ofthese materials were sourced from various vendors, others wereformulated using proprietary mixtures of commercially availablemonomers, and some were molecularly synthesized. The acrylate polymermaterials is tested include the following: 2,2,2-trifluoroethylacrylate;75% 2,2,2-trifluoroethylacrylate/25% C₁₉ diol diacrylate; 50%acrylonitrile/50% C₂₀ diol diacrylate; dimerol diacrylate; 50%acrylonitrile/50% C₁₉ diol diacrylate; Erpol 1010 diacrylate; 50% hexanediol diacrylate/50% C₂₀ triol diacrylate; 50%2,2,2-trichloroacrylate/50% C₁₉ diol diacrylate; 75%2,2,2-trifluoroethylacrylate/25% C₁₉ diol diacrylate; 50% octane dioldiacrylate/50% 2-cyanoethyl acrylate; 67%2,2,2-trifluoroethylacrylate/33% C₂₀ triol triacrylate; lauryl acrylate;trimethylolpropane triacrylate; ethoxyethoxy ethyl acrylate;pentaerythritol tetraacrylate; neo-pentyl glycol diacrylate; octyldecylacrylate; tetraethylene glycol diacrylate; tripropylene glycoldiacrylate; octane diol diacrylate; 1,8-alkoxylated aliphatic acrylate;decanediol diacrylate; ethylene glycol diacrylate; iso-bornyl acrylate;butanediol diacrylate; 93% hexane diol diacrylate/7% KENSTAT q100; 95%hexane diol diacrylate/5% chlorinated polyester diacrylate;trimethylolpropane ethoxylate triacrylate; trimethylolpropanepropoxylate triacrylate; neo-pentyl glycol propoxylate diacrylate;bisphenol A ethoxylate diacrylate; alkoxylated aliphatic diacrylateester; 50% 2-cyanoethyl acrylate/50% C₁₉ diol diacrylate; 92%trimethylolpropane triacrylate/8% acrylonitrile; 50% iso-bornylacrylate/50% pentaerythritol triacrylate; 83% trimethylolpropanetriacrylate/7% FLUORAD FXS89; 50% acrylonitrile/50% trimethylolpropanetriacrylate; 70% trimethylolpropane triacrylate/30% acrylonitrile; 40%FLUORORAD FX189/60% trimethylolpropane triacrylate; 70% hexane dioldiacrylate/30% iso-bornyl acrylate; 50% hexane diol diacrylate/50%iso-bornyl acrylate; 12.5% aliphatic urethane triacrylate/87.5% hexanediol diacrylate; 31% KENSTAT q100/69% C₁₄ diol diacrylate; 69% C₁₄ dioldiacrylate/31% acrylonitrile; 80% C₁₄ diol diacrylate/20% KENSTAT q100;94% trimethylolpropane propoxylate triacrylate/6% KENSTAT q100; 50%trimethylolpropane triacrylate/50% acetonitrile; 70% trimethylolpropanetriacrylate/30% acetonitrile; 88% phenol ethoxylate monoacrylate/12%acetonitrile; 80% C₁₄ diol diacrylate/20% acetonitrile; 80% C₁₄ dioldiacrylate/20% KENSTAT q100; 12% trimethylolpropane triacrylate/88%iso-bornyl acrylate; 69% C₁₄ diol diacrylate/23% KENSTAT q100/8%trimethylolpropane triacrylate; 33% acetonitrile/33% polyamineacrylate/34% iso-bornyl acrylate; 75% aliphatic amine acrylate/17%KENSTAT q100/8% trimethylolpropane triacrylate; 80% C₁₄ dioldiacrylate/20% FLUORAD FX189; 80% phenol ethoxylate monoacrylate/−20%pentaerythritol triacrylate; 80% hexane diol diacrylate/20%acrylonitrile; 70% hexane diol diacrylate/15% acrylonitrile/15%trimethylolpropane triacrylate; propoxylated glycerine triacrylate;ethoxylated trimethylolpropane triacrylate; caprolactone acrylate; 90%alkoxylated trifunctional acrylate/10% beta-carboxyethyl acrylate; 90%polyethylene glycol 200 diacrylate/10% pentaerithritol di tritetraacrylate; 75% hexane diol diacrylate/25% KenReact LICA 44; 50%pentaerythrytol tetraacrylate/50% hexane diol diacrylate;pentaerythritol polyoxyethylene petraacylate; tetrahydrofurfurylacrylate; 25% KENSTAT q100/75% hexane diol diacrylate; 50%tetrahydrofurfuryl acrylate/50% polyethylene glycol 200 diacrylate; 88%tetrahydrofurfuryl acrylate/12% trimethylolpropane triacrylate; 88%caprolactone acrylate/12% trimethylolpropane triacrylate; EBECRYL 170;80% EBECRYL 584/20% beta-carboxyethyl acrylate; 88% tetrahydrofurfurylacrylate/12% beta-carboxyethyl acrylate; 88% EBECRYL 170/12%beta-carboxyethyl acrylate; aliphatic polyesther hexaacrylate oligomer;aliphatic urethane diacrylate; tripropylene glycol diacrylate; hexanediol diacrylate; 88% iso-bornyl acrylate/12% beta-carboxyethyl acrylate;90% polyethylene glycol 200 diacrylate/5% iso-bornyl acrylate/5%pentaerythritol triacrylate; 82% polyethylene glycol 200 diacrylate/12%hexane diol diacrylate/6% trimethylolpropane triacrylate; 75%tetrahydrofurfuryl acrylate/15% hexane diol diacrylate/5%trimethylolpropane triacrylate/5% oligomer; 44% polyethylene glycol 200diacrylate/44% acrylonitrile/12% hexane diol diacrylate; 70% C₁₄ dioldiacrylate/30% aliphatic urethane acrylate oligomer; trimethylolpropaneethoxylate triacrylate; and 50% PHOTOMER 6173/50% hexane diol diacrylate50%. Notes: KENSTAT q100 and KenReact LICA 44 are trade names of KenrichCorp.; FLUORAD FX189 is a trade name of 3M Industrial Chemical Corp.;EBECRYL 170 and EBECRYL 184 are trade names of UCB Radcure Corp.; andPHOTOMER 6173 is a tradename of Henkel Corp.

[0059] Several acrylate polymers were identified as candidate materialsfor AC voltage capacitor designs. These are low dissipation factor (DF)polymers with DF <0.01, i.e., <1%, and dielectric constant (1c) in therange 2.5<κ<4.0.

[0060] PP-acrylate hybrid films were produced using a production-sizevacuum metallizing chamber that was retrofitted to allow deposition ofthe acrylate coatings in line with the metallization process. PP filmsof 6 μm, 8 μm, 12 μm, and 19 μm were first treated with a gas plasma andthen coated with acrylate polymer films with thicknesses of about 0.2 to1.0 μm. Dielectric characterization of small area stamp capacitors, withPP (control) and hybrid films, showed that the PP-acrylate films hadsuperior current carrying capability, higher resistance to degradationfrom partial discharges (corona), and breakdown voltage equal to orhigher than PP films of equal thickness.

[0061] An additional benefit that was unexpected was improved corrosionresistance of the metallized aluminum electrodes when deposited on theacrylate coating rather than on the PP film. This is very significantbecause it allows the use of thinner aluminum layers, which increasesthe self-healing properties of the hybrid film capacitors.

[0062] Several full-size capacitor designs were produced and tested.Capacitors with ratings of 8 μF/330 VAC, 0.1 μF/1200 VDC, and 0.1μF/2000 VDC were built using acrylate-coated 6 μm, 12 μm, and 19 μm PPfilms, respectively. Short-term current, humidity, and breakdown voltagetests showed that the acrylate-PP hybrid capacitors followed theperformance of the small area stamp capacitors. That is, the hybrid filmcapacitors had significantly higher performance than that of the PPcontrol capacitors. In a critical high dV/dt test that applied 5000 Vpulses with a rise time of 1000 V/50 ns, the 0.1 μF/2000 V capacitorsout-performed by far commercial capacitors that used double metallizedPET film electrodes (for higher current carrying capacity) and equalthickness dielectric. The commercial capacitors degraded or failed after2400 pulses, while there was no degradation in the 19 μm hybrid filmcapacitors. In fact, the 12 μm hybrid film capacitors (0.1 μF/1200 V)that were less than half the volume of the 19 μm capacitors were alsosuperior to the commercial capacitors when tested under the sameconditions. A comparison of 12 μm PP lighting ballast capacitors with 12μm acrylate-PP film capacitors using 3000 V pulses showed no degradationin the hybrid film capacitors, while the performance of conventionalcapacitors from two different manufacturers varied from significantdegradation to complete failure.

[0063] Large quantities of acrylate hybrid films were coated,metallized, and wound into capacitors using conventional windingequipment. The acrylate-PP film handled as well as any capacitor filmthrough the various process steps. Since the acrylate polymer can bedeposited in-line with the metallization, and in the same vacuumchamber, added labor cost is minimal. The base acrylate monomers areavailable at a cost of $3/lb to $4/lb and at a thickness of about 1 μm,added material cost will be minimal.

[0064] A. Hybrid Film Production Process Development

[0065] Acrylate-PP hybrid films were produced using 6 μm, 8 μm, 12 μm,and 19 μm PP films that were readily available. Rolls of film 32 cm widewere used that were at least a few thousand feet long (typical size forsmall metallizing runs). The apparatus employed in the coating processis shown schematically in FIG. 1. The liquid monomer was pumped from thereservoir 40 that was located outside the vacuum chamber 12 into theflash evaporator 28. The liquid monomer was atomized into microdropletswith the use of the ultrasonic atomizer 42 that was positioned on top ofthe evaporator 23. The evaporator 23 was held at a temperature which wasabove the boiling point of the liquid, but below its decompositionpoint. This caused the monomer to flash evaporate before it cured. Themolecular vapor exited at supersonic speeds and condensed on the film 20that was in intimate contact with the chilled rotating drum 16. Thecondensed thin film deposit then moved in front of the electron beam gun30 where it was polymerized.

[0066] Examples of suitable acrylates useful in the practice of theinvention include iso-bornyl acrylate, hexane diol diacrylate, andtripropylene glycol diacrylate that were formulated for fast cure,proper viscosity, and good adhesion to the PP film. These acrylates allhave a dissipation factor of less than 0.01.

[0067] Several large rolls of 6 μm and 8 μm PP film were coated undervarious conditions until a set of parameters that produced a well-cured,uniform coating was obtained. The major process parameters are asfollows:

[0068] 1. Drum Temperature: Good films may be made within thetemperature range from room temperature to −20° C. The lower temperatureappeared to increase some-what the condensation rate with some monomers,but not with others. It is estimated that the residence time of the filmon the drum was not sufficient to transfer much of the drum temperatureto the top surface of the film. As a result, the difference in drumtemperature did not have a major affect on the monomer condensationrate.

[0069] 2. Drum Speed: Good films were produced with drum surface speedsanywhere in the range of 50 to 1000 feet/min (25 to 500 cm/sec).

[0070] 3 Radiation Dose: The accelerating potential was 12 KeV and thecurrent delivered to the monomer was about 2 mA for curing monomer films1 μm thick.

[0071] 4. Final Monomer Mixture: Most of the monomer mixtures that wereprocessed produced good quality coatings when considering parameterssuch as uniformity, degree of cure, and adhesion to the PP film. Oneexample of a suitable monomer mixture comprised a mixture of 70%hexanediol diacrylate, 20% iso-bornyl acrylate, and 10% tripropyleneglycol diacrylate.

[0072] 5. Plasma Treatment Prior to Acrylate Deposition: The PP film wasplasma-treated prior to the deposition of the monomer vapor for thefollowing reasons:

[0073] a. The PP film surface has absorbed oxygen and moisture thatinterferes with the polymerization of the acrylate monomer. Oxygen is afree radical scavenger that neutralizes free radicals created by theelectron beam, thus inhibiting the polymerization process.

[0074] b. The plasma treatment etches and cleans the film surface fromlow molecular weight residue created by the corona treatment process.This improves the wetting of the monomer to the film, resulting in amore uniform coating

[0075] 6. Plasma Treatment After Deposition of the Acrylate Polymer:This is a post-treatment that was used to reduce the static charge onthe coated film and also complete the polymerization process on theacrylate surface. This process step reduced the “stickiness” on theacrylate-PP wound roll.

[0076] B. Evaluation of the Hybrid Films Using Small Area Films

[0077] The hybrid films were first evaluated using small area stampcapacitors, and then full-size-wound capacitors were fabricated forfurther evaluation. Stamp capacitors were used to evaluate breakdownstrength, current carrying capacity, partial discharge (corona)degradation of the polymer films, and corrosion resistance of themetallized electrodes.

[0078] 1. Dielectric Constant and Dissipation Factor

[0079] The acrylate coating, due to its low thickness, had minorinfluence on both the and DF of the hybrid films. Depending on theparticular hybrid, the dielectric constant of the acrylate-PP films wasup to 7% higher than that of plain PP film.

[0080] 2. DC Breakdown Strength

[0081] This measurement was made to ensure that the hybrid film had abreakdown strength at least as high as that of the original PP film,plus an additional amount due to the acrylate coating. The breakdownmeasurements on the films were done using a dry double-metallized andnon-contact measurement technique that was performed at high vacuum(<10⁻⁴ Torr), to eliminate partial discharges and surface flashovers.The breakdown system was built in a turbomolecularly pumpedstainless-steel chamber. Metallization masks were designed andfabricated that allowed metallization of small area (about 1 squareinch) stamp capacitors for the breakdown measurements. The electrodecontact was made to metallized pads that were outside the active area toprevent film damage. The voltage was ramped at about 500 V/sec. Forevery breakdown measurement that is reported below, at least 18 stampcapacitors were tested to ensure that the data is statisticallysignificant.

[0082] The results of the DC breakdown tests of 6 μm and 8 μm filmscoated with about 0.5 μm of acrylate polymer are shown in Table I. Theacrylate polymer is produced by electron beam curing of a 70% hexanedioldiacrylate, 20% iso-bornyl acrylate, and 10% tripropylene glycoldiacrylate monomer film deposited in the vacuum using the experimentalapparatus described in FIG. 1. The PP film was first plasma-treatedusing an argon gas plasma and the acrylate monomer was flash evaporatedon the treated surface. TABLE I THICKNESS BREAKDOWN FILM (μm) VOLTAGE(KV) Control PP 6 3.87 PP/plasma/acrylate 6.5 4.25 Control PP 8.0 4.84PP/plasma/acrylate 8.5 5.34 Control PP 12.0 5.30 PP/plasma/acrylate 12.56.26

[0083] The breakdown voltage of the control and acrylate hybrid filmswas measured by metallizing one square inch electrodes on opposing sidesof the films. Each measurement represents an average of at leasteighteen individual breakdown measurements. The data in Table I showthat the acrylate-coated PP films have a breakdown strength that ishigher than the control PP film by about 10%. Given that the coatingthickness is about 0.5 μm, the breakdown strength of the acrylate-PPfilms is equal to or higher than PP films. Similar measurements onsingle acrylate layers have shown that the breakdown strength of 1 μmthick films is 20 to 24 KV/mil, which is equal to or higher than that ofPP film.

[0084] 3. Current-Carrying Capacity

[0085] It is well-known that due to the low melting point of PP film,metallized PP capacitors become quite unreliable at high currentapplications due to thermal damage of the termination. The currentgenerates I²R losses (R=Equivalent Series Resistance, ESR) at thetermination, which raises the temperature, which in turn damages thetermination and increases the ESR. This process eventually causes acatastrophic failure. Life test data showed that many conventional PPcapacitor designs have marginal current carrying performance. For thisreason, in some high current designs, double metallized paper or doublemetallized PET film that has a higher melting point than PP is used tocarry the current. These PET/PP designs are inefficient and result inexpensive capacitor products.

[0086] To simulate the current flow from the sprayed termination to themetallized film, and the ability of the film to carry high currentswithout thermal damage, a simple test was developed to measure themaximum power that a metallized film can dissipate prior to a thermalfailure. Current was forced through a section of metallized film and thepower (I×V) was increased until the dissipated heat forced the film tofail. The fixture had ½ inch wide contact electrodes which were placed 5inches apart. AC voltage was applied to the electrodes and the currentthrough the circuit was recorded. The voltage was raised until the powerloss thermally degraded the metallized film to the point of failure. Inthe films that were tested, the failure was an open circuit caused bymelting of the PP film at some point close to the middle of the 5 inchstrip. Table II shows the average power to failure (eight samples weretested for each condition), for both coated and uncoated 8 μm thickpolypropylene. The acrylate polymer was produced by electron beam curingof a 70% hexanediol diacrylate, 20% iso-bornyl acrylate, and 10%tripropylene glycol diacrylate monomer film deposited in the vacuumusing the experimental apparatus described in FIG. 1. The PP film wasfirst plasma-treated using an argon gas plasma and the acrylate monomerwas flash-evaporated on the treated surface. TABLE II POWER IMPROVEMENTOVER FILM TYPE (Watts) CONTROL PP (%) Control PP 13.5 — PP/plasma/0.2 μm14.0 3.7 acrylate PP/plasma/0.4 μm 14.8 9.6 acrylate PP/plasma/0.6 μm15.6 15.6 acrylate PP/plasma/0.8 μm 19.4 43.7 acrylate PP/plasma/1.0 μm22.0 63.0 acrylate

[0087] The results show that the coated films have a higher thermalcapacity that varies from 4% to 63%. The variation is due mostly to thethickness of the acrylate coating.

[0088] It is interesting to note that although the coated films failedat significantly higher power levels, they did not deform and shrink asmuch as the plain films. The higher thermal capacity of the hybrid filmswas a key objective of this work and it is clear that thin coatings ofthe high temperature acrylate films can have a significant impact in thethermal properties of PP.

[0089] 4. Resistance to Degradation from Partial Discharges (Corona)

[0090] One of the common mechanisms of failure in high voltage (V>300VDC) film capacitors is damage to the polymer dielectric from partialdischarge activity (corona) in the capacitor windings. The partialdischarges or corona pulses are generated in inter-electrode areas thathave large enough air gaps to sustain a certain level of ionization. Thehigh temperature corona pulses, although they physically move around,can degrade the polymer dielectric and cause a breakdown. Dry metallizedcapacitors are particularly susceptible to corona damage, especially atthe outer and innermost turns which are looser than the rest of theroll. Metallized capacitors that have electrodes with reasonably highresistance (3 to 8 ohm/sq have good self-healing properties, and thecorona-induced clearings will result in some capacitance loss with nofurther damage to the capacitor. Capacitors with electrodes of 2 to 3ohm/sq have poorer clearing properties. The resulting high levels ofcorona can lead to major capacitance loss, increased dissipation factor,higher leakage current, and often catastrophic failures

[0091] Resistance to the damaging thermal effects of the corona pulsesincreases as the thermal stability of the polymer film increases.Control polyvinylidene difluoride (PVDF) and polypropylene (PP) filmswere tested along with PVDF/plasma/acrylate and PP/plasma/acrylatehybrid films for resistance to corona degradation. The acrylate polymerwas produced by electron beam curing of a 70% hexanediol diacrylate, 20%iso-bornyl acrylate, and 10% tripropylene glycol diacrylate monomer filmdeposited in is the vacuum using the experimental apparatus described inFIG. 1. The PP film was first plasma-treated using an argon gas plasmaand the acrylate monomer was flash-evaporated on the treated surface.

[0092] The acrylate-PVDF hybrid was included because PVDF film is thehighest energy density capacitor dielectric that is commerciallyavailable, and an acrylate-PVDF hybrid may present some unique productopportunities. Furthermore, the PVDF film has about the same meltingpoint as PP. The PVDF stamp capacitors were liquid impregnated andtested at 1200 VAC. The PP capacitors were dry and tested at 350 VAC.This test was an accelerated corona test, where the level of partialdischarges reflected either poor impregnation (impregnated cap), orloose outer turns in a dry capacitor. To date, several stamp capacitorshave been tested, and the results are shown in Table III. TABLE III TIMETO CAPACITOR BREAK- NUMBER FILM TYPE DOWN (min.) 1 Control PVDF, 12 μm3.0 2 Control PVDF, 12 μm 3.8 3 Control PYDF, 12 μm 5.1 4 Control PVDF,12 μm 3.8 5 PVDF/plasma/1 μm 47.8 acrylate) 6 PVDF/plasma/1 μm 29.0acrylate) 7 PVDF/plasma/1 μm 32.0 acrylate) 8 PVDF/plasma/1 μm 33.8acrylate) 9 Control PP, 8 μm 19.0 10 PP/plasma/1 μm acrylate 133.0

[0093] AS seen in Table III, the hybrid films had about one order ofmagnitude longer time to failure. This can raise the reliability levelof the capacitors significantly. In the case of a pulse-type capacitor,it may represent a large number of additional pulses prior to failurefrom corona degradation.

[0094] 5. Corrosion Resistance

[0095] Capacitance loss due to electrode corrosion is the most commonfailure mechanism in metallized capacitors. The corrosion resistance ofaluminum electrodes deposited on control PP and PP/plasma/acrylate filmswas tested. The corrosion stability of aluminum metallized PP andPP/plasma/acrylate films was tested by measuring the change inelectrical resistance of a 5 inch by 1 inch metallized strip, afterexposure in an temperature/humidity environment of 70 C185% RH. The 5inch strips were cut out of large bobbins of material which were alsoused to make full size capacitors. In order to assure that a goodelectrode contact was made at each measurement (especially as theelectrode starts to corrode), a thicker aluminum pad was deposited atboth ends of the 5 inch strip.

[0096] The control PP and hybrid films were produced from the same rollof PP film by plasma treating and acrylate coating half the roll (32 cmwide and about 10,000 feet long) and then metallizing the entire roll ina single metallizing run. FIG. 2 shows the change in electricalresistance as a function of time for 6 μm control PP film (Curve 52)metallized with aluminum side by side with PP/plasma/1 μm acrylate film(Curve 50). The acrylate polymer was produced by electron beam curing ofa 70% hexanediol diacrylate, 20% iso-bornyl acrylate, and 10%tripropylene glycol diacrylate monomer film deposited in the vacuumusing the experimental apparatus described in FIG. 1. The PP film wasfirst plasma-treated using an argon gas plasma and the acrylate monomerwas flash-evaporated on the treated surface. The data shows that themetallized aluminum electrodes on the hybrid films have superiorcorrosion resistance.

[0097] The surface properties of the polymer film play a major role inthe structural and chemical stability of the metallized electrodes.Several studies have been conducted in the past that show that surfacetreatment of the polymer films prior to the metallization improves theadhesion and corrosion resistance of the aluminum electrodes. Filmcross-linking, polar chemical functionalities and lack of low molecularweight material are parameters that favor the acrylate surface. Inaddition, PP film, as a rule, is always corona or flame treated prior tometallization. This is done in-line with the film manufacturing process.Subsequent exposure to ambient conditions leads to moisture adsorptionby the carboxyl, carbonyl, and hydroxy groups formed by the treatmentprocess. The adsorbed moisture will react with the aluminum deposit anddegrade its chemical stability.

[0098] The above tests indicate that the acrylate hybrid films haveunique surface properties surface that enhance the chemical resistanceof evaporated aluminum metal coatings.

[0099] C. Evaluation of Hybrid Films Using Full Size Capacitors

[0100] Full size capacitors based on an 330 VAC design were fabricatedusing control 8 μm PP films and PP/plasma/0.7 μm acrylate hybrid film.The acrylate polymer was produced by electron beam curing of a 70%hexanediol diacrylate, 20% iso-bornyl acrylate, and 10% tripropyleneglycol diacrylate monomer film deposited in the vacuum using theexperimental apparatus described in FIG. 1. The PP film was firstplasma-treated using an argon gas plasma and the acrylate monomer wasflash-evaporated on the treated surface. The capacitors were wound insolid hex-cores and they had a cylindrical shape.

[0101] Full size capacitors were built and life-tested for electrodecorrosion resistance under the same conditions as the small area films.Instead of measuring the capacitance change, due to the short testperiod, the optical density of the electrodes was measured. In thismanner, the aluminum oxidation could be detected even when the electrodewas conducting, which would not result in a capacitance loss. Theoptical density was initially measured on the bobbins that were used tomake the capacitors. After the test, the capacitors were unwound and theoptical density of the metallized film was measured at the outer part ofthe roll at some fixed distance away from the end of the winding. Theresults of this test are shown in Table IV. Four sets of capacitors weremade using control PP and PP/plasma/0.7 μm acrylate hybrid filmmetallized with thin aluminum (optical density 1.25 and 1.51, denoted as(1) in Table IV) and thicker aluminum (optical density 1.68 and 1.76,denoted as (2) in Table IV). TABLE IV CHANGES IN OPTICAL DENSITY OFALUMINUM METALLIZED CAPACITOR FILMS OF PP AND PP/PLASMA/0.7 μm ACRYLATEFILMS, DUE TO CORROSION IN A 70° C. AND 85% RH ENVIRONMENT. OPTICALPERCENTAGE OPTICAL DENSITY, DENSITY, CHANGE OVER MATERIAL 0 HOURS 214HOURS 214 HOURS Hybrid (1) 1.25 0.98 −21.60 Control PP (1) 1.51 0.55−63.58 Hybrid (2) 1.68 1.38 −17.86 Control PP (2) 1.76 1.41 −19.89

[0102] The results in Table IV indicate that the corrosion performanceof control PP and acrylate-PP hybrid films, when wound into capacitorsrolls, is basically the same as that of the small area films (hybrid (1)is the same film material tested in small area samples, see FIG. 2).That is, metallized capacitors wound with acrylate hybrid films sufferless electrode corrosion than similar parts produced with metallizedcontrol PP films.

[0103] This is a significant advantage for the hybrid films. Theimproved corrosion resistance of the metallized electrodes will allowuse of this product in applications is where the capacitors are exposedto higher humidity and temperature.

[0104] A high frequency, high current/low voltage test was developed toevaluate the current carrying capacity of the capacitor termination. Thecurrent through the capacitors was altered by varying the voltageamplitude of the high frequency signal. After testing several capacitorsto determine a set of conditions that would degrade the capacitortermination by some measurable amount, both plain PP and hybrid filmcapacitors were tested. The test data as shown in Table V demonstratesthat capacitors built with the acrylate-PP hybrid film dielectrics havesuperior parametric stability than sister units produced with control PPfilm. TABLE V DISSIPATION FACTOR AND ESR OF CAPACITORS BUILT WITH PP ANDACRYLATE-PP HYBRID FILM. THEY ARE TESTED AT 40 KHz BEFORE AND AFTER THEHIGH CURRENT TEST. UNIT # CAP (μf) DF (%) ESR (m ohms) INITIAL VALUESControl PP 8.92 1.13 5.0 Hybrid (2) 8.86 0.65 2.9 AFTER 10 MIN. 25 KHz50 A PEAK AT 60 V PEAK Control PP 8.76 1.15 5.20 Hybrid (2) 8.82 0.693.10 AFTER 5 ADD'L MIN. 25 KHz 75 A PEAK AT 85 V PEAK Control PP 7.4011.00 59.20 Hybrid (2) 8.59 0.71 3.30

[0105] The current-carrying performance of the full size capacitors isin line with the current or power dissipation performance of the smallarea metallized films (Table IV). Furthermore, it is interesting to notethat the full size hybrid film capacitors were fabricated with Hybrid(2) film (see Table IV), which did not have the highest powerdissipation performance. This suggests that capacitors built withoptimized hybrid films could have far superior performance to thecapacitors that were tested.

[0106] Capacitors with 0.1 μF capacitance were fabricated, using 19 μmPP/plasma/1.0 μm acrylate (referred to as Hybrid A in Table VI) and 12μm PP/plasma/1.0 μm acrylate (referred to as Hybrid B in Table VI).These capacitors were arbitrarily rated 0 μF/2000 VDC and 0.1 μF/1200VDC, respectively. The acrylate polymer was produced by electron beamcuring of a 70% hexanediol diacrylate, 20% iso-bornyl acrylate, and 10%tripropylene glycol diacrylate monomer film deposited in the vacuumusing the experimental apparatus described in FIG. 1. The PP film wasfirst plasma-treated using an argon gas plasma and the acrylate monomerwas flash-evaporated on the treated surface.

[0107] The capacitors were compared with state-of-the-art high currentcommercial capacitors, which although they had a rating of 0.1 μF/11200V, they also utilized 19 μm PP dielectric and double metallizedpolyester (PET) film electrodes, to carry the high current. Thesecapacitors were about the same size as the Hybrid (A) film 0.1 μFmetallized capacitors and more than double the volume of the Hybrid (B)0.1 μF capacitors.

[0108] All capacitors were tested using 5,000 V pulses with a dV/dt of1000 V/ns. The capacitance and ESR were measured using a conventionalelectronic capacitance bridge. The initial capacitance and ESR arecompared to the after test capacitance and ESR in Table VI below. TABLEVI VOLTAGE AND CURRENT CARRYING COMPARISON OF STATE OF THE ARTCOMMERCIAL PP CAPACITORS AND CAPACITORS BUILT WITH ACRYLATE HYBRID FILMSOF THE PRESENT INVENTION Initial ESR Final ESR Initial Capacitance @ 100KHz Final Capacitance @ 100 KHz @ 100 KHz (nf) (milliohms) @ 100 KHz(nf) (milliohms) Commercial Capacitor 98.53 5 63.72 2115 98.34 6 33.918294 98.36 5 98.11 5 101.72 7 33.97 7768 97.69 5 62.94 2050 97.30 651.81 3249 98.35 7 51.30 3112 98.11 3 98.00 6 Average Change: −37.4%+3319.5 mΩ Present Invention, 19 μm Hybrid (0.1 μF/2000 V) 102.13 8101.90 9 103.43 9 103.37 9 102.83 6 102.83 9 103.30 4 103.28 4 103.77 4103.76 4 95.08 4 95.08 4 103.61 9 103.59 9 103.43 8 103.43 9 AverageChange: −0.05% +0.62 mΩ Present Invention, 12 μm Hybrid (0.1 μF/1200 V)100.06 5 99.79 10 99.82 11 81.89 395 101.30 9 82.70 302 101.70 11 94.80229 100.30 10 86.10 901 102.60 14 80.00 411 100.80 10 87.10 439 101.9010 99.90 45 Average Change: −11.9% +331.5 mΩ

[0109] It can be seen that the 0.1 μF/2000 VDC (Hybrid (A)) design,outperforms the equal size commercial design by a very large margin. Infact, the data shows that all but two of the commercial capacitorsfailed, while none of the capacitors prepared by the process of thepresent invention had any significant degradation. With regard to the0.1 μF/1200 V (Hybrid (B)) capacitors prepared in accordance with thepresent invention (that have less than half the size of the commercialcapacitors), the data show that al-though these capacitors degraded, onaverage, they performed better than the state of the art commercialcapacitors.

[0110] II. Food Packaging Films

[0111] In packaging applications, barrier films are of particularinterest for separating products such as food and electronic componentsfrom environments that reduce product life. Such barrier films may beachieved either using films in which the barrier is built into the filmor by applying a coating to a film. The barrier coating may be eithertransparent, comprising oxide materials for example, or opaque orsemi-transparent, comprising metals for example.

[0112] Extension of shelf life at a competitive cost has been acontinuing challenge for film producers, metallizing houses andlaminators. Specifically, for food applications, the metallized filmacts as (a) an oxygen and moisture barrier, to help keep food fresh andcrisp, (b) a light barrier, to reduce rancidity in fatty foods, and (c)an aroma barrier, to keep the original flavor intact. Additionalrequirements include processability of the metallized film, abrasionresistance, mechanical robustness, reliability and cost.

[0113] A. Opaque Metallized Barrier Films

[0114] The barrier properties of the metallized aluminum films dependalmost entirely on the integrity of the metallized aluminum layer. Thequality of the metallized layer depends on defects on the surface of thepolymer film. Gross defects are manifested mainly in the form ofpinholes in the metallization that are visible with the opticalmicroscope. Pinhole defects are usually traced to particles on the filmsurface and abrasion of the aluminum on film fibrils and other surfaceprotrusions caused by antioxidants and slip agents. These features dotthe surface landscape of the films and are characteristic of aparticular film vendor and manufacturing process. Although the numberand size of pinholes can dominate the transmission of oxygen through thefilm, there are additional microstructure defects that determine theultimate barrier possible for a given film. Microstructure defects canhave several forms that include micropin-holes, microcracks, andmicroareas of very thin aluminum. These defects can be traced toproperties of the substrate film that involve the basic film chemistryand manufacturing process. Parameters such as the molecular weight ofthe resin used to make the film additives to the resin, processconditions and surface treatment of the film. Improved barrierperformance can be achieved by minimizing both gross and microstructuredefect density on the film surface.

[0115] Several materials were investigated for use in the production ofhigh barrier metallized films. One prerequisite is that the monomer andof course the polymer coatings must have a low amount of volatilecomponents, to assure that the package material does not produce anodor. In order to qualify polymer materials, a standard “aroma test” wasconducted, and many of the polymer materials that were tested passed thetest. A deodorized version of tripropylene glycol diacrylate wasarbitrarily chosen to conduct oxygen barrier tests.

[0116] For opaque high oxygen barrier packages (potato chip bags), theindustry standard is polypropylene film (PP) metallized with a thinlayer of aluminum (Al). The barrier material development included thefollowing designs of metallized PP and acrylate hybrid films.

[0117] 1. PP/Plasma/Al/Acryl/Plasma

[0118] The acrylate polymer was produced by electron beam curing of atripropylene glycol diacrylate monomer film deposited in the vacuumusing the experimental apparatus described in FIG. 1. The thickness ofthe acrylate polymer was about 1.0 μm thick. The PP film was firstplasma-treated using a gas plasma produced by an Ar/N₂ gas mixture witha 10%/90% ratio, and aluminum was applied on top of the plasma-treatedsurface. The acrylate polymer was applied on top of the thin aluminummetal to prevent pinhole formation from abrasion of the film surface onrollers in the vacuum chamber and in subsequent processes such asslitting and laminating. The structure is depicted in FIG. 3D.

[0119] The plasma treatment functionalization of PP film resulted inhigher nucleation rates of the aluminum and finer crystal structure. Theformation of finer aluminum crystallites was confirmed using X-raydiffraction (XD) analysis. Specifically, the metallized films weretested using a Scintag XRD system, which revealed that the aluminum2.344 CPS peak for the plasma-treated film was at least three times lessintense and about twice as broad as the non-treated metallized film.Also, transmission electron microscopy (TEM) and electron diffractionanalysis conducted on a Hitachi TEM showed that the crystals in thealuminum deposited on the plasma-treated PP film were much smaller withvery broad diffraction rings (when compared to the aluminum deposited oncontrol PP). The finer aluminum crystallites result in a more flexiblemetal film that resists microcracking during stretching. This wasmeasured by monitoring the electrical resistance of a metallized filmwhile the film was elongated at a fixed speed, using acomputer-controlled system. In a metallized film, microcracking of themetal layer will result in an increase of its electrical resistance.FIG. 4 shows the change in resistance versus percent elongation for acontrol PP/aluminum film (Curve 54) and PP/plasma/aluminum (Curve 56).As can be seen, the aluminum deposited on the functionalizedplasma-treated surface has significantly more microcracking resistancethan the control.

[0120] 2. PP/Plasma/Acryl/Al

[0121] The acrylate polymer was produced by electron beam curing of atripropylene glycol diacrylate monomer film deposited in the vacuumusing the experimental apparatus described in FIG. 1. The thickness ofthe acrylate polymer was about 1.0 μm thick. The PP film was firstplasma-treated using a gas plasma produced by an Ar/N₂ gas mixture witha 10%/90% ratio, and the acrylate monomer was flash-evaporated on thetreated surface. The acrylate polymer was applied below the aluminumlayer to smooth out the rough surface of the PP film and to also providea thermally and mechanically superior substrate. As shown in thecapacitor section above, this results in higher corrosion stability ofthe aluminum layer. The structure is depicted in FIG. 3B.

[0122] In addition, the surface of the PP film contains low molecularweight material composed of PP, degraded PP, and slip agents added tothe film during manufacture. It was discovered that these lowermolecular weight species reflow when exposed to the heat generated fromthe evaporation sources and the condensation of the aluminum deposit.The reflow prevents nucleation of aluminum atoms on the surface of thesecites, thus creating pinholes that degrade the moisture and oxygenbarrier properties. This effect is eliminated or minimized when the hightemperature acrylate polymer is deposited on the PP surface and thenmetallized.

[0123] 3. PP/Plasma/Acryl/Al/Acryl/Plasma

[0124] The acrylate polymer was produced by electron beam curing of atripropylene glycol diacrylate monomer film deposited in the vacuumusing the experimental apparatus described in FIG. 1. The thickness ofthe acrylate polymer was about 1.0 μm thick. The PP film was firstplasma-treated using a gas plasma produced by an Ar/N₂ gas mixture witha 10%/90% ratio, and the acrylate monomer was flash-evaporated on thetreated surface. This is a combination of the two above systems. Othercombinations with a second aluminum layer are also possible, but lesscost effective. The structure is depicted in FIG. 3C.

[0125] In order to make the hybrid film production economically moreattractive, lower grades of PP films (lower cost) may be used that whenmetallized have Oxygen Transmission Rates (OTR) in the range of 3 to 10,versus higher quality films that have OTRs in the range of 1 to 3. Thedata in Table VII below show that the OTR of control films (PP/Al) canbe reduced from 6.17 to as low as 0.07 using the hybrid film designsdescribed above. All OTR and MVTR barrier measurements reported hereinwere performed by a third party, using commercial equipment. Consideringthat the hybrid films can be produced in the same metallizationequipment that the control films are made and with little additionalcost, the OTR values of the hybrid films represents an unprecedentedlevel of improvement, and are far superior to commercially availablefilms. TABLE VII OXYGEN TRANSMISSION RATE (OTR) OF CONTROL METALLIZED PP(PP/Al) AND ACRYLATE-PP HYBRID FILMS OTR FILM TYPE cm³/100 in²/day PP/Al(control) 6.17 PP/plasma/Al/acryl/plasma 0.59 PP/plasma/acryl/Al 2.54PP/plasma/acryl/Al/acryl/plasma 0.07

[0126] It should be noted that the polypropylene was plasma-treated inthe case that the acrylate polymer was deposited directly on thepolypropylene (configurations 2 and 3 above). The plasma treatment wasfound to be advantageous for two reasons, (a) it removes adsorbed airand moisture from the surface of the PP film, which enhances the degreeof cross-linking on the PP/acrylate interface, and (b) it functionalizesthe PP film, which facilitates wetting of the acrylate monomer. Severalgases were used for the plasma treatment operation that included Ar, N₂,O₂, Ne, CO₂, and mixtures thereof. Ar and N₂ mixed in a 10%/90% ratioproduced a very effective mixture as measured by wetting and uniformityof the polymer layer, and so did mixtures with 5% O₂. Another effectivegas mixture incorporates 99.9% Ne and 0.1% Ar, and mixture of that with10% N₂, or 5% of O₂, or 5% of CO₂. The long lived metastable levels inthe Ne atom enhance the ionization level of the mixture and increasesignificantly the ionization current and surface treatment. Anothereffective mixture is 96% CF₄ and 4% O₂.

[0127] It was also found that at times, after the hybrid film was woundinto a roll, a certain level of “blocking” or tackiness developed,because the back side of the polypropylene film had a soft copolymerlayer deposited on it that is used to thermally seal the plastic bag.The tackiness was a result of the combined effect of partial cure on thesurface of the acrylate coating and electrostatic charge (electrons)that can be trapped in deep and shallow potential wells in the acrylatecoating.

[0128] It was found that the blocking could be eliminated with theaddition of a second cross-inking and discharge station, after theacrylate polymer is formed in the first station. The second crosslinking station can be another lower energy electron beam, or a plasmatreatment station. An electron beam with 600 to 1200 eV energy was usedto add surface cure and discharge the surface of the acrylate polymer.At this voltage level, most polymer materials have a secondary electronemission coefficient that is higher than 1.0, thus leading to a surfacedischarge, because for every incoming electron more than one electronsare emitted from the surface. A plasma treatment station accomplishesthe same result, although the mechanism is different. The radiation in aplasma discharge includes ions and UV radiation. The UV radiationincludes very energetic (15 eV) vacuum UV photons that enhance surfacecross-linking, while the ions help discharge the film surface.

[0129] An alternate method of preventing blocking of the film in theroll is to use monomer formulations that include small quantities ofadditives such as mono-acrylate monomers that cannot fully cross-link,or that are partially conductive. This leads to the formation of a“slippery” and/or antistatic acrylate surface that inhibits blocking andstatic charge formation.

[0130] B. Transparent Ceramic-Coated Barrier Films

[0131] Environmental constraints are leading to the replacement ofcertain chlorine-containing high barrier polymer films, with polyesterfilms coated with transparent barrier layers of inorganic materials suchas aluminum oxide and silicon oxides (SiO_(x), where x=1 to 2).

[0132] Some food manufacturers have been slow in accepting the highercost coated transparent barrier films due to the higher price of thepolyester film that appears to be necessary for the production oftransparent high barrier coatings. For this, several attempts have beenmade to produce lower cost transparent barrier using a polypropylene(PP) substrate films. This work has shown that the low melting point PPpolymer films are in general a poor substrate for the deposition of thehigher melting point inorganic coatings. Thus, ceramic-coatedtransparent barrier films that are presently in the market place utilizepolyester film as a substrate that has superior thermomechanicalproperties to PP.

[0133] The present invention addresses the low cost production of highbarrier metallized films as well as high barrier transparent PP films,using a combination of vacuum polymer coating and surface modificationby plasma treatment. PP film was plasma-treated using a 90% N₂-10% Argas mixture and a low aroma acrylate monomer was deposited andcross-linked with electron beam radiation. The accelerating voltage istypically 10 to 12 KeV for about 1.0 μm of acrylate coating. The current(or number of electrons) varies with web speed. For a one foot wide webmoving at 100 to 200 ft/min, 5 to 10 mA of current was used.

[0134] The transparent films employed in the practice of the presentinvention include aluminum oxide, silicon oxides (SiO_(x), where x=1 to2), tantalum oxide, aluminum nitride, titanium nitride, silicon nitride,silicon oxy-nitride, zinc oxide, indium oxide, and indium tin oxide. Thethickness of the ceramic coating may range from about 5 to 100 nm foruse as a barrier layer. Preferably, the best barrier properties areobtained in the lower portion of the thickness range. The ceramiccoating may be deposited by electron beam evaporation.

[0135] Two types of transparent barrier films were produced; one using athin inorganic film of SiO_(x), where x=1 to 2, and Al₂O₃. The SiO_(x)film was deposited using plasma deposition from a silane gas mixture andthe Al₂O₃ film was deposited using electron beam evaporation. In thisexperiment, the objective was to see if the thermomechanically superioracrylate hybrid film had improved barrier properties, rather thanoptimizing the barrier properties to a minimum. The results as shown inTable VIII indicate that the oxygen and moisture barrier properties ofthe transparent PP/plasma/acrylate/ceramic film are superior to thecontrol PP/ceramic film. TABLE VIII OXYGEN (OTR) AND MOISTURE (MVTR)TRANSMISSION RATE OF CONTROL PP/CERAMIC AND PP/PLASMA/ACRYLATE/CERAMICHYBRID FILMS MVTR OTR FILM TYPE g/100 in²/day cm³/100 in²/day PP/SiO_(x)(control) 0.25 9.2 PP/Al₂O₃ (control) 21.4 PP/plasma/acryl/SiO_(x) 0.102.5 PP/plasma/acryl/Al₂O₃ 4.0

[0136] In packaging applications, it is important that the barrierproperties of a film are maintained through the process of processingthe film into a bag. This process exposes the film to some elongationand stretching as the film is pulled over a forming collar that forcesthe flat film to conform into a cylindrical opening that is sealed firston the side, then the bottom and on the top after food is automaticallyinserted in the bag. If conventional metallized PP film is elongated,then the aluminum layer can easily crack, leading to loss of barrier.Since barrier properties as a function of elongation is difficult tomeasure, the cracking of the aluminum as a function of elongation wasmeasured indirectly by measuring the resistivity of the aluminum. FIG. 5shows that the aluminum deposited on PP (Curve 58) has far inferiormechanical properties that of a PP/plasma/aluminum/acrylate/plasmahybrid film (Curve 60), using 90%/N₂-10% Ar gas plasma and adhesionpromoted deodorized tripropyleneglycol diacrylate monomer.

[0137] Further investigation on the structure of the aluminum usingX-ray diffraction analysis, transmission electron microscopy, andelectron diffraction analysis show that the aluminum formed on thecontrol PP surface has large crystals, while aluminum deposited in thehybrid film has much finer crystal structure. Such crystal structure incombination with superior aluminum adhesion to the nitrided PP surfaceand the protective acrylate coating, minimize the formation ofmicrocracks and preserve the original barrier properties of the film.Several packaging trials conducted by a leading manufacturer of snackfoods have shown that bags produced with a monoweb ofPP/plasma/aluminum/acrylate. The acrylate polymer was produced byelectron beam curing of a tripropylene glycol diacrylate monomer filmdeposited in the vacuum using the experimental apparatus described inFIG. 1. The thickness of the acrylate polymer was about 1.0 μm thick.The PP film was first plasma-treated using a gas plasma produced by anAr/N₂ gas mixture with a 10%/90% ratio, and aluminum was applied on topof the plasma-treated surface. The acrylate polymer was applied on topof the thin aluminum metal to prevent pinhole formation from abrasion ofthe film surface on rollers in the vacuum chamber and in subsequentprocesses such as slitting and laminating.

[0138] The hybrid film maintained low MVTR and OTR, while the controlPP/Al film can increase by about an order of magnitude. Table IX showsOTR and MVTR data following film production, after printing andfollowing bag production. TABLE IX OXYGEN (OTR) AND MOISTURE (MVTR)TRANSMISSION RATE OF PP/PLASMA/ ALUMINUM/ACRYLATE/PLASMA HYBRID FILMS ATDIFFERENT PRODUCTION STAGES OF A SNACK FOOD BAG FILM TYPE MVTRPP/plasma/metal/acrylate/plasma g/100 in²/day OTR cm³/100 in²/day FilmProduction 0.004 0.34 Printing 0.003 0.35 Bag Production 0.01 1.41

[0139] III. Films for Printing Applications

[0140] In the above description of packaging applications, thePP/plasma/metal/-acrylate/plasma hybrid film can be printed directlyafter production without any special preparations The printing industryemploys a broad range of printing inks that include three maincategories; water-based, solvent-(oil) based, and 100% solids. Althoughthese inks are formulated to wet different kinds of surfaces, surfacesare often formulated to be wetted by a particular ink process andequipment that a food manufacturer has in place. The acrylate hybridfilms can be designed to be wetted by various ink chemistries. Thehydrophobic/philic and oliophobic/philic surface wetting properties weremeasured using the solutions shown in Table X. A drop of each of thesolutions in Table X was deposited on the surface of an acrylatepolymer, obtained by electron beam curing of the acrylate monomer. Ifthe liquid drop of a particular composition (1-6) does not wet thesurface within the specified period (see foot notes in Table X), thenthe surface is considered phobic for that liquid. TABLE X SOLUTIONCHEMISTRY USED FOR WETTABILITY TESTS, NUMBER “1” IS MOST PHOBIC OilRating Number Composition 1 Kaydol (mineral oil) 2 65/35Kaydol/n-hexadecane 3 n-hexadecane 4 n-tetradecane 5 n-dodecane 6n-decane 30 seconds wetting period Water Rating Number % iso-Propanol %Water 1 2 98 2 5 95 3 10 90 4 20 80 5 30 70 6 40 60 10 seconds wettingperiod

[0141] About 100 different acrylate formulations were produced andtested for basic wetting properties (see list under Capacitors, above).The results in FIG. 6 show that hybrid films can be formulated withspecific surface wetting characteristics to accommodate a broad range ofink chemistries. Some examples of acrylate polymer formulations andtheir corresponding wetting properties are shown in Table XI. TABLE XIOIL AND WATER WETTING PROPERTIES OF ACRYLATE HYBRID FILMS(PP/plasma/Acrylate), SEE TABLE X FOR WETTING RATING NUMBERS. Oil WaterAcrylate Polymer 1 6 neo-pentyl glycol diacrylate 5 6 octyldecylacrylate 3 6 octane diol diacrylate 1,8 3 4 akoxylated aliphaticdiacrylate ester 3 6 decanediol diacrylate 1 3 93% hexane dioldiacrylate/7% KENSTAT q100

[0142] In addition to printing applications, the surface wettingfunctionalization of the PP/acrylate films can be useful in otherapplications where surface wetting or lack of it is important. Forexample, a Teflon-like surface that repels water can have manyindustrial and commercial applications. An oliophilic surface could beused in film/foil capacitor applications where the wound rolls have tobe thoroughly impregnated with a dielectric oil-based fluid.

[0143] IV. Magnetic Tapes

[0144] The need for increasingly higher density storage media, thewidespread use of narrow format video tapes, and the development ofdigital audio and video tape drives are leading to shorter wavelengthrecording, which allows higher data storage per unit length of tape.Such recording tapes utilize a thin metal magnetic coating that can beobtained by metal evaporation, sputtering and plating techniques. Thisapproach eliminates the polymer binder that is utilized in conventionalmagnetic coatings resulting in ferromagnetic layers with highersaturation magnetization, which is suitable for higher storagedensities.

[0145] When short wavelength recording is used, dropout or the loss ofinformation is is much higher for a given spacing between the recordingmedium and the magnetic head. The spacing loss is represented by theformula: 54.6 d/w, where d is the tape to head distance, and w is therecording wavelength Therefore, if the surface of the magnetic medium,or even the opposite side are rough, loss of information will occur asthe tape is running in front of the head. For this reason, films thatare used for metallized magnetic tapes are quite costly because thesurface roughness of both the front and the back surfaces have to bewell-controlled to assure low degree of roughness. It should be notedthat if the roughness of the back side is too low, then the tape canbind and be damaged.

[0146] An additional problem with thin metal tapes is damage of themagnetic due to abrasion resistance and environmental corrosion. Tapemanufacturers have devised different ways of dealing with theselimitations. Radiation-curable materials are commonly used to coat boththe front and back surface of the substrate tape and often inorganicadditives for controlled roughness and antistatic properties are used onthe back side coating (U.S. Pat. Nos. 5,068,145; 4,670,340; 4,720,4214,781,965; 4,812,351; 4,67,083; and 5,085,911). The existing methods forapplying the radiation-curable coatings use conventional applicationsmethods (roll coating, casting, and solution based) under atmosphericconditions, and the cure utilizes UV or electron beam, performed in airor in a nitrogen environment.

[0147] In the present invention, acrylate coatings are vapor-depositedin vacuum which allows for thinner, pinhole free and more uniformcoatings, in-line with the deposition of the magnetic coating. Acrylatepolymer films deposited on one surface of polyester film can flattensurface irregularities by one to two orders of magnitude, depending onthe thickness of the acrylate layer. For example, a 2.0 μm acrylatecoating on a surface with 400 nm asperities reduces the average surfaceroughness to less than 50 nm. The acrylate polymer was produced byelectron beam curing of a hexane diol diacrylate monomer film depositedin the vacuum using the experimental apparatus described in FIG. 1. Thesubstrate film was first plasma-treated using a gas plasma produced byan Ar/N₂ gas mixture with a 10%/90% ratio. If a flatter film is usedwith 50 nm asperities, then a 1.0 μm or less of hexane diol diacrylateacrylate layer results in a virtually flat film surface with a surfaceroughness of the order of 1 to 2 nm.

[0148] As mentioned above, a surface with a controlled roughness on thebackside of the magnetic tape is often desirable to minimize frictionover rollers. A hybrid film with a controlled microroughness wasproduced in the following manner. After the deposition of aradiation-curable monomer, an electron beam is used to polymerize thethin monomer film. For a film thickness of 1.0 μm, a beam with anaccelerating voltage of about 8 to 12 KeV may be used. If the beamvoltage is reduced considerably, the penetrating depth of the electronsis reduced proportionally. For example, an accelerating voltage of about600 to 1000 electron volts will cure the surface of the acrylate film,thus creating a thin skin. The heat produced in the exothermicpolymerization reaction and the shrinkage of the acrylate film duringpolymerization causes the skin to wrinkle in a very controlled way.

[0149] A cross-section of the wrinkled surface 62 a is shown in FIG. 7A.The periodicity and depth of the wrinkles can be controlled based on thethickness of the monomer film, accelerating voltage, current (number ofelectrons) and film shrinkage, that can be varied by changing theacrylate chemistry. Acrylate materials with film shrinkage greater than5% are desirable for this process, with shrinkage levels of about 15%being preferable. Such monomers include materials such aspentaerythritol triacrylate, hexane diol diacrylate, trimethylolpropanetriacrylate, pentaerythritol tetraacrylate, ditrimethylolpropanetetraacrylate, dipentaerythritol pentaacrylate, tripropylene glycoldiacrylate and tetraethylene glycol diacrylate. After the production ofthe wrinkled surface, a second higher voltage electron beam is used topolymerize the whole monomer film thickness.

[0150] The reduced voltage may be obtained by placing a smaller electronbeam gun 230, shown in phantom in FIG. 1, between the flash evaporator28 and the radiation curing means 30.

[0151] The roughened surface 62 a shown in FIG. 7A can be used in thebackside of a magnetic tape to minimize abrasion and facilitate freemovement over rollers. Such a surface may also be useful in film/foilcapacitors to facilitate impregnation of capacitor rolls with adielectric fluid.

[0152] V. Optical Filters

[0153] Polymer films with optical effects such as color shift andholographic images are used in various applications that includewrapping films and tamper proof and anti-counterfeiting medallions. Twosuch acrylate hybrid films were produced in a one-step low costmanufacturing process.

[0154] (A) The wrinkled film described above, when metallized after thesecond electron beam cure (FIG. 7B), creates brilliant color shiftingreflections if the wrinkles size is small enough to interfere with theambient light. Although in this work only a uniform interference patternwas produced across the film surface, color shifting images can beproduced by patterning the wrinkle area 62 b and by varying the wrinklesize within an image. This can be accomplished by replacing the wideelectron beam curtain with a single point electron beam that can becomputer controlled to move in the X-Y axis and also have variablecurrent by modulation of the Z axis.

[0155] (B) A uniform color shifting film 64 was produced using arelatively flat substrate film 120, or a relatively rough film that wasflattened with an acrylate coating. The configuration of the colorshifting film (shown in FIG. 8), was as follows: polyester/plasma/metal1/acrylate 1/metal 2/acrylate 2/plasma. The acrylate polymer wasproduced by electron beam curing of a hexane diol diacrylate monomerfilm deposited in the vacuum using the experimental apparatus describedin FIG. 1. The polyester substrate film was first plasma-treated using agas plasma produced by an Ar/N₂ gas mixture with a 10%/90% ratio. Metal1 was an aluminum film 124 that is highly reflective with an with anoptical density of about 3 to 4. Acrylate 1 was a thin acrylate coating122 with a thickness that was 1 wavelength of visible light (0.4 to 0.7μm). Metal 2 was a thin semi-transparent layer 124′ with optical densityof about 0.5 to 1.5. Acrylate 2 is an optional protective coating 122′that can have any thickness (typically 0.5 to 3 μm) that providesadequate abrasion resistance. Such hybrid film results in a uniformbright color that shifts to higher or lower frequency color as theviewing angle changes.

[0156] Light beam 66 is incident on the color shifting device 64, and aportion passes through the semi-transparent metal layer 124′ and isreflected as beam 68 a by the opaque metal layer 124. Another portion ofincident Light beam 66 is reflected as beam 68 b by the semi-transparentmetal layer 124′.

[0157] The above color shift hybrid film could be made to color shift onboth sides. That is, it could also color shift through the transparentpolyester substrate film, if the color shifting stack is symmetrical.Such structure would be as follows: Polyester/semi-transparentaluminum/¼λ acrylate/reflective aluminum/¼λ acrylate/semi-transparentaluminum. Given the above hybrid film design, it will also be relativelyeasy to someone skilled in the art to produce a color shift pigment byreleasing the color shifting stack from the polyester substrate. Thiscan be accomplished by coating the polyester substrate with a layer ofpolystyrene. The color shifting stack is then deposited on thepolystyrene, which can be dissolved in a solvent to produce symmetriccolor shifting flakes, that can be reduced to a finer color shiftingpigment.

[0158] VI. Flexible Cables

[0159] In electrical cables, such as those used in aircraft to directpower to various parts of the airplane, arcing can occur that createsso-called “electrical tracking”, which may result in fires. The use ofthese films in military aircraft has been banned by the U.S. Navy, andlater the U.S. Army and Air Force, due to poor electrical trackingresistance that has resulted in several documented cases of fires, andeven loss of military aircraft. One solution has been to replace thethermally and mechanically robust polyimide films that have poortracking properties with polytetrafluoroethylene-(“Teflon”) coated wire,which has excellent arc tracking properties. However, the Teflon coatingis rather soft and it can be easily damaged. A solution to this problemis to clad Teflon on polyimide film (see Chemfab entry in Table XII,below). This hybrid film has tracking properties similar to Teflon, withsuperior thermal and mechanical properties.

[0160] In the present invention, reduction of electrical tracking isaccomplished with the use of a hybrid film that utilizes a combinationof a Teflon-like vacuum polymer coating and surface modification byplasma treatment. An electrical arc tracking test was developed tosimulate the performance of cable insulation. A high voltage, highimpedance AC voltage (5 to 15 KV), was applied between two flat metalplates that were 5 inches long, 2 inches wide and {fraction (1/16)} inchthick. The test samples (in the form of 1 to 6 mil thick films) werewrapped around the plates and the two plates were fixtured against eachother along their length on one plane. The gap between the plates wasadjusted to about 1 mm and the voltage was raised until an arc initiatedat one of the two opposing sets of corners in the two plates that wasintentionally exposed (not covered with the test films) to allow the arcto strike. The polymer films start to burn as soon as the arc strikes.The rate of arc movement (film burning) accurately replicates theability of the material to arc-track. The system was tested using knownmaterials such as Teflon and Kapton films that have low and high arctracking rates, respectively. TABLE XII ARC TRACKING RESISTANCE OFSEVERAL POLYMER FILMS LAYERS/ TRACKING DIS- FILM THICKNESS TANCE (inch)PBO/Fluoroacrylate, 12 μm 4/1 mil no arc PTFE 4/1 mil  <1/32   Chemfab,Style 072992A 2/2 mil  <1/32   Polyester (PET) 8/0.5 mil  <1/32  Polypropylene (PP) 8/0.5 mil  4/32 Kapton/Fluoroacrylate, 12 μm 4/1 mil 4/32 ETFE (Tefzel film) 4/1 mil 12/32 PFCB 2/0.8 mil (best of 3) 30/32PIBO 4/1 mil 32/32 Polycarbonate (PC) 8/0.5 mil 40/32 PBO 4/1 mil 60/32Kapton 2/2 mil 60/32

[0161] A functionally similar hybrid film was produced by depositing afluorine-containing acrylate film on one or both sides of Kapton (DuPontfilm) and PBO (Dow Chemical film). Table XII shows a clear relationshipbetween the thickness of a fluoroacrylate coating on 25 μm of PBO andKapton films. The fluoroacrylate coating was composed of a 25%:75%mixture of hexane diol diacrylate (HDODA):perfluoroalkysulfonamide, andthen cross-linked with an electron beam, using the apparatus describedin FIG. 1. For the PBO-coated films, the improvement in arc trackingparalleled the coating thickness until a critical thickness of about 12μm was reached. Beyond that point, the tracking resistance was equal tothat of PTFE. At 23 μm of coating thickness, it was not possible tostrike a sustainable arc, thus giving an apparent arc trackingresistance better than that of PTFE. Table XIII shows trackingresistance of is the 12 μm coated PBO and Kapton films, along withseveral other commercial films, that were tested under the sameconditions, and are included as benchmarks to provide a comparison meansfor the arc tracking performance of the fluoroacrylate coatings. TABLEXIII ARC TRACKING DISTANCE OF FLUOROACRYLATE-COATED PBO AND KAPTON FILMSFLUOROACRYLATE DISTANCE ARC BASE FILM COATING THICKNESS TRAVELED (inch)PBO none 60/32 PBO  6 μm 44/32 PBO 9.5 μm  32/32 PBO 12 μm  <1/32   PBO23 μm  <1/32   Kapton 10 μm 40/32 Kapton 12 μm  4/32

INDUSTRIAL APPLICABILITY

[0162] The hybrid polymer films comprising a polymer film, aplasma-treated surface, a vacuum-deposited, radiation-curable acrylatepolymer with a plasma-treated surface, are expected to find a variety ofuses in food packaging films, thin film metallized and foil capacitors,flexible electrical cables, metal evaporated magnetic tapes, opticallyvariable films, and other applications that utilize films such as PP andPET.

[0163] While certain embodiments of the present invention have beendisclosed herein, it will be readily apparent to those skilled in thisart that various changes and modifications of an obvious nature may bemade, and all such changes and modifications are considered to fallwithin the scope of the invention as defined by the appended claims.

What is claimed is:
 1. A hybrid polymer film, comprising: a firstpolymer film having a plasma-treated surface; and a second polymer filmhaving first and second surfaces, the first surface of the secondpolymer film being disposed along the first plasma-treated surface ofthe first polymer film.
 2. The hybrid polymer film of claim 1, whereinthe second polymer film is formed from a radiation-cured monomer film.3. The hybrid polymer film of claim 2, wherein the second polymer filmis an acrylate polymer film.
 4. The hybrid polymer film of claim 3,wherein the second polymer film is a fluorinated acrylate polymer film.5. The hybrid polymer film of claim 2, wherein the second surface of thesecond polymer film is a plasma-treated surface.
 6. The hybrid polymerfilm of claim 1, wherein the second surface of the second polymer filmis a plasma-treated surface.
 7. The hybrid polymer film of claim 1,wherein the first polymer film is formed from a thermoplastic polymer.8. The hybrid polymer film of claim 1, wherein the first polymer film isformed from a thermoset polymer.
 9. The hybrid polymer film of claim 1,wherein the first plasma-treated surface of the first polymer film has amicroroughness greater than about 1 nanometer.
 10. The hybrid polymerfilm of claim 1, further comprising a metal film having first and secondsurfaces, the first surface of the metal film disposed along the secondsurface of the second polymer film.
 11. The hybrid polymer film of claim5, further comprising a metal film having first and second surfaces, thefirst surface of the metal film disposed along the plasma-treatedsurface of the second polymer film.
 12. The hybrid polymer film of claim10, further comprising a third polymer film having first and secondsurfaces, the first surface of the third polymer film, being disposedalong the second surface of the metal film.
 13. The hybrid polymer filmof claim 11, further comprising a third polymer film having first andsecond surfaces, the first surface of the third polymer film beingdisposed along the second surface of the metal film.
 14. The hybridpolymer film of claim 12, wherein the third polymer film is formed froma radiation-cured monomer film.
 15. The hybrid polymer film of claim 1,wherein the first polymer film has a second plasma-treated surface. 16.The hybrid polymer film of claim 15, further comprising a third polymerfilm disposed along the second plasma-treated surface of the firstpolymer film.
 17. The hybrid polymer film of claim 16, wherein the thirdpolymer film is an acrylate polymer film.
 18. The hybrid polymer film ofclaim 17, wherein the third polymer film is a fluorinated acrylatepolymer film.
 19. The hybrid polymer film of claim 1, further comprisinga ceramic layer disposed along the second surface of the second polymerfilm.
 20. The hybrid polymer film of claim 19, wherein the ceramic layercomprises a material selected from the group consisting of aluminumoxide, a silicon oxide, tantalum oxide, aluminum nitride, siliconnitride, silicon oxy-nitride, zinc oxide, indium oxide, and indium tinoxide.
 21. The hybrid polymer film of claim 20, wherein the ceramiclayer comprises a material selected from the group consisting ofaluminum oxide and a silicon oxide.
 22. A hybrid polymer film,comprising: a first polymer film having a plasma-treated surface; and afirst metal film having first and second surfaces, the first surface ofthe first metal film being disposed along the first plasma-treatedsurface of the first polymer film.
 23. The hybrid polymer film of claim22, wherein the first metal film is formed from a material selected fromthe group consisting of aluminum, zinc, nickel, cobalt, iron, iron onaluminum, zinc on silver, zinc on copper, zinc on aluminum,nickel-cobalt alloy, and nickel-cobalt-iron alloy.
 24. The hybridpolymer film of claim 22, wherein the first polymer film is formed froma thermoplastic polymer.
 25. The hybrid polymer film of claim 22,wherein the first polymer film is formed from a thermoset polymer. 26.The hybrid polymer film of claim 22, wherein the first plasma-treatedsurface of the first polymer film has a microroughness greater thanabout 1 nanometer.
 27. The hybrid polymer film of claim 22, furthercomprising a second polymer having first and second surfaces, the firstsurface of the second polymer film disposed along the second surface ofthe first metal film.
 28. The hybrid polymer film of claim 27, whereinthe second polymer film is formed from a radiation-cured monomer film.29. The hybrid polymer film of claim 28, wherein the second polymer filmhas a plasma-treated surface.
 30. The hybrid polymer film of claim 28,wherein the second polymer film is an acrylate polymer film.
 31. Thehybrid polymer film of claim 27, further comprising a second metal filmhaving, first and second surfaces, the first surface of the second metalfilm disposed along the second surface of the polymer film.
 32. Thehybrid polymer film of claim 29, further comprising a second metal filmhaving first and second surfaces, the first surface of the second metalfilm disposed along the plasma-treated surface of the second polymerfilm.
 33. The hybrid polymer film of claim 31, further comprising athird polymer film disposed along the second surface of the second metalfilm.
 34. The hybrid polymer film of claim 33, wherein the third polymerfilm is formed from a radiation-cured monomer film.
 35. The hybridpolymer film of claim 32, further comprising a third polymer filmdisposed along the second surface of the second metal film.
 36. Thehybrid polymer film of claim 35, wherein the third polymer film isformed from a radiation-cured monomer film.
 37. The hybrid polymer filmof claim 36, wherein the third polymer film has a plasma-treatedsurface.
 38. The hybrid polymer film of claim 33, wherein the firstmetal film comprises a reflective film of aluminum, the second polymerfilm comprises an acrylate polymer having a thickness that is ¼wavelength of visible light, the second metal film comprises asemi-transparent film of aluminum, and the third polymer film comprisesan acrylate polymer.
 39. A method of forming a hybrid polymer film, themethod comprising the steps of: plasma-treating a first surface of afirst polymer film to form a first plasma-treated surface of the firstpolymer film, and forming a second polymer film on the plasma-treatedsurface of the first polymer film.
 40. The method of claim 39, whereinthe step of forming a second polymer film includes the steps of:depositing a monomer film on the first plasma-treated surface of thefirst polymer film; and radiation-curing the monomer film.
 41. Themethod of claim 40, further comprising the step of: plasma-treating asurface of the second polymer film to form a plasma-treated surface ofthe second polymer film.
 42. The method of claim 41, further comprisingthe step of: depositing a metal film on the plasma-treated surface ofthe second polymer film.
 43. The method of claim 42, further comprisingthe step of: forming a third polymer film on a surface of the metalfilm.
 44. The method of claim 43, wherein the step of forming the thirdpolymer film includes the steps of: depositing a monomer film on thesurface of the metal film; and radiation-curing the monomer film. 45.The method of claim 39, further comprising the step of: plasma-treatinga second surface of the first polymer film to form a second plasma-treated surface of the first polymer film.
 46. The method of claim 45,further comprising the step of: forming a third polymer film on thesecond plasma-treated surface of the first polymer film.
 47. The methodof claim 46, wherein the step of forming the third polymer film includesthe steps of: depositing a monomer film on the second plasma-treatedsurface of the first polymer film; and radiation-curing the monomerfilm.
 48. The method of claim 39, further comprising the step ofdepositing a ceramic layer on the surface of the second polymer film.49. A method of forming a hybrid polymer film, the method comprising thesteps of: plasma-treating a first surface of a first polymer film toform a plasma-treated surface of the first polymer film; and forming ametal film on the plasma-treated surface of the first polymer film. 50.The method of claim 49, further comprising a step of: forming a secondpolymer film on a surface of the metal film.
 51. The method of claim 50,wherein the step of forming the second polymer film includes the stepsof: depositing a monomer film on the metal film; and radiation-curingthe monomer film.
 52. The method of claim 51, further comprising thestep of: plasma-treating a surface of the second polymer film to form aplasma-treated surface of the second polymer film.
 53. The method ofclaim 52, further comprising the step of: depositing a second metal filmon the plasma-treated surface of the second polymer film.
 54. The methodof claim 53, further comprising the step of: forming a third polymerfilm on the surface of the second metal film.
 55. The method of claim54, wherein the step of forming the third polymer film includes thesteps of: depositing a monomer film on the surface of the second metalfilm; and radiation-curing the monomer film.
 56. The method of claim 55,further comprising the step of: plasma-treating a surface of the secondpolymer film to form a plasma-treated surface of the second polymerfilm.